Digital Subscriber Line (DSL) is a well known access technology that uses existing 2-wire copper telephone wiring (also known as unshielded twisted pair UTP) to deliver high-speed data services to homes and businesses. DSL technology has become popular with both subscribers and Internet service providers because the service uses the customer's existing phone line and typically does not require an additional phone line. In addition, the signaling used by some DSL technology is above that used by plain old telephone service (POTS). Hence, this allows an “always-on” Internet access while still providing use of the phone line. Current ADSL technology offers users a choice of speeds up to about 8 Mbps. This is much faster than a standard 56 Kbps dial-up modem.
There are many types of DSL, generically these have come to be known by the designation xDSL. For any given line, for a particular type of xDSL, the maximum xDSL speed is limited by the frequency response of the channel which is a function of the distance between the subscriber and the central office (CO) and the filtering placed on the line, and by the noise conditions on the line which are primarily a function of crosstalk from other signals in the cable and ingress from AM radio transmitters. On long loops speed frequency response and noise conditions of the channel bound the upper rate. On shorter loops the xDSL technology and its allocated bandwidth may be bit rate determining factor.
Asymmetric Digital Subscriber Line (ADSL) is one of the xDSL technologies that provide more bandwidth in one direction than the other, typically downstream from the central office to the subscriber. Very high bit-rate Digital Subscriber Line (VDSL) is a particular type DSL that may be configured as an asymmetric or a symmetric serive. It delivers from 13 to 52 megabits per second downstream bandwidth and 1.5 to 13 megabits per second upstream. VDSL may be implemented using single carrier or DMT based modulation. ADSL uses a form of modulation known as discrete multi-tone modulation (DMT). DMT is method of modulation that divides the available frequency range into sub-channels or tones, the number of sub-channels depends on the particular implementation.
ITU-T Recommendation G.992.1 also known as G.dmt, is a form of ADSL technology, using DMT modulation, that offers up to 8 MBPS downstream bandwidth, 1.544 MBPS upstream bandwidth. ITU-T Recommendation G.992.2 is also known as G.lite, is a form of ADSL technology, using DMT modulation, that offers up to 1.5 MBPS downstream bandwidth, 384 KBPS upstream. The rates mentioned above are not caps on the achievable data rate in these technologies but reflect rates discussed in the ITU standards documents.
The deployment of xDSL services in the copper loops helped service providers in providing new services and creating new revenues. However, the introduction of these services also created new challenges to the service providers in the maintenance and diagnostics area. Traditional methods of determining that a twisted pair is suitable for POTs service are not adequate for these higher frequency services. The rate determining factors of channel frequency response and noise can change over time, and require monitoring to ensure adequate service. These challenges include how to provide effective and low cost maintenance operation for the new services.